<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .on{
      width: 100%;
        height: 200px;
        background-color: brown;
    }
    .in{
        width: 100%;
        height: 0;
        transition: height 0.3s;
    }
    .on:hover  .in{
        height: 200px;
        background-color: chocolate;
    }
</style>

<body>
    
    <div class="on">

        <div class="in"></div>
    </div>
</body>
</html>